JSON encoding for TransactionHintsBag type hierarchy

[greenhat]

This PR implements JSON encoding for the hints type hierarchy, including SigmaBoolean and proof tree types used in sigma protocol. Besides that, it swaps the external Scalar with its wrapped version Wscalar.
To implement property-based testing all of the above-mentioned types got Arbitrary implementation.

TODO:

• serde attributes/types;
• implement todo!;
• compare JSON to node's implementation;
• custom Arbitrary impl for *SecretProof types (image == unchecked_tree);
• make an issue for Cthreshold::polynomial checked ctor Check polynomial in Cthreshold* has correct size #579

[coveralls]

Pull Request Test Coverage Report for Build 2434368031

• 209 of 228 (91.67%) changed or added relevant lines in 19 files are covered.
• 29 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.2%) to 87.836%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ergotree-interpreter/src/json/hint.rs	20	21	95.24%
ergotree-interpreter/src/sigma_protocol/prover/prover_result.rs	3	4	75.0%
ergotree-interpreter/src/sigma_protocol/sig_serializer.rs	9	10	90.0%
ergotree-interpreter/src/sigma_protocol/prover.rs	5	7	71.43%
ergotree-interpreter/src/sigma_protocol.rs	3	6	50.0%
ergotree-ir/src/chain/json/sigma_protocol.rs	26	37	70.27%

Files with Coverage Reduction	New Missed Lines	%
ergotree-interpreter/src/sigma_protocol.rs	1	61.54%
ergotree-ir/src/serialization/types.rs	6	82.7%
ergotree-ir/src/chain/json/ergo_box.rs	7	94.26%
ergotree-ir/src/chain/ergo_box.rs	15	91.02%

Totals
Change from base Build 2361989666:	0.2%
Covered Lines:	15316
Relevant Lines:	17437

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💛 - Coveralls

[kettlebell]

Just a couple of minor comments but otherwise it looks good to me!